1. Field of the Invention
The invention relates to a method for determining an actuating quantity of a mechanically commutated DC motor (commutator motor). The invention additionally relates to a device for carrying out the method, and to the use of a modified commutator motor for such a method.
2. Description of the Background Art
Commutator motors are oftentimes used as actuating motors within an actuating device for or in a motor vehicle. Such actuating devices are, for example, an electric power window regulator, an electric seat adjustment mechanism, an electric door or sunroof closing mechanism.
In these actuating devices (actuating systems), precise knowledge of the instantaneous actuation position, travel distance, and/or actuation speed is required, in particular to be able to travel precisely to an end position that is to be reached for an actuating operation, and to be able to detect hazard situations, such as a pinch event, in a timely manner. The actuation position and quantities derived therefrom, such as the actuation speed and the actuation distance traveled, are jointly referred to below as “actuating quantities.” Each of these actuating quantities can be defined in an equivalent manner with respect to the motor, to the motor vehicle part to be moved, or to another component of the actuating device moved during the actuation process. For example, the actuation position in a power window regulator can be specified in an equivalent manner by the angle of rotation of the motor shaft or the window position. Similarly, the actuation speed can be specified in an equivalent manner by the speed of the motor shaft or the travel speed of the window, etc.
In methods operating without sensors—such as is known from for example, DE 10, 2006 049 123 A1, which corresponds to US Publication No. 20100315032, actuating quantities of the above-mentioned type are typically determined by counting the motor current ripples. In this context, (motor current) ripple designates a characteristic ripple (i.e., periodic, pulsating variations) of the motor current that is caused by the commutation of the DC motor.
However, error-free counting of the motor current ripples is not possible in all phases of a typical actuation process. Thus, a typical actuation process is made up of an initial startup phase, an equilibrium phase (steady state), a freewheeling phase, and a final braking phase.
During the startup phase, the motor speed settles to a stable final speed. In the subsequent equilibrium phase, this final speed and hence also the frequency of the current ripples is approximately constant.
The freewheeling phase is initiated in that both terminals of the commutator motor are switched to ground in order to terminate the actuation process. The freewheeling phase lasts for the duration of this switching process (typically approximately 3-4 msec). As a result of the switching process, either no motor current or an extremely irreproducible motor current flows during the freewheeling phase, with essentially unchanged actuation speed of the motor. As soon as both motor contacts are switched to ground in a stable manner, the freewheeling phase transitions to the final braking phase. In the braking phase, the electric motor, which is short-circuited through ground, is operated as a generator and is braked by the circulating current thus produced.
As a result of the collapsing motor current, it is impossible to count the current ripples in the freewheeling phase, in particular. Ripple counting is indeed possible in principle during the startup and braking phases, but suffers an increased risk of error on account of comparatively irregular current relationships.
In conventional methods for ripple counting, this is why counting errors in ripple counting typically occur during the freewheeling phase in particular, but to a lesser degree in the startup phase and the braking phase as well. Such counting errors occur primarily because current ripples in the current signal are not detected and thus are essentially “lost.” Furthermore, however, counting errors can also occur because externally caused disturbances in the motor current behavior are incorrectly identified as current ripples. Both types of counting errors result in errors determining the actuating quantities. Especially during calculation of the actuation position, these errors can accumulate in an unfavorable manner during the course of several successive actuation processes, and thus can significantly impair the function of the actuating device under certain circumstances.
A method is known from DE 20 2004 010 211 U1, which corresponds to U.S. Pat. No. 7,741,800, which is incorporated herein be reference, and in which, in order to correct counting errors in the ripple count, pattern recognition is first carried out, during the course of which characteristic amplitude differences are identified in the successive ripples within a motor cycle. As a result of this identification of the individual ripples in the motor cycle, “lost” or incorrectly identified ripples can be detected and the count result can be corrected accordingly. It is additionally proposed in DE 20 2004 010 211 U1 to also ascertain the motor position by means of an electromechanical motor model in parallel with the ripple counting.
It is known furthermore from DE 41 35 873 C2, which corresponds to U.S. Pat. No. 5,432,421, and which is incorporated herein by reference, to produce an irregular ripple pattern—within one ripple cycle—through modification of a commutator motor in order to be able to detect not just the travel position or travel distance or the travel speed by means of the ripple pattern, but also the direction of travel.